1. Field of the Invention
The present invention relates generally to a washer-type pressure sensor. More specifically, the invention relates to a washer-type pressure sensor suitable for monitoring internal pressure in an engine combustion chamber.
2. Description of the Background Art
Washer-type pressure sensors have been used for monitoring internal pressure in engine combustion chambers, for monitoring combustion in the engine combustion chambers and for detecting engine knocking and so forth. One typical washer-type pressure sensor is disclosed in Japanese Utility Model First (unexamined) Publication (Jikkai) Showa 62-46346. According to the disclosure of Jikkai Showa 62-46346, the washer-type pressure sensor comprises a ring-shaped center electrode. A pair of piezoelectric elements are provided on both sides of the center electrode. An upper and lower pressure plate are provided outside of respective piezoelectric elements. The center electrode, the piezoelectric elements, and the pressure plates are housed in an annular metal case.
Another known type of washer type pressure sensor has upper and lower pressure plates, the upper pressure plate being formed of an electrically conductive material and serving as an external electrode. An insulating tube is disposed within an aligned hole of the center electrode, the piezoelectric elements and the pressure plate. The upper pressure plate has a cylindrical extension extending through the aligned hole. The cylindrical extension is insulated from the center electrode, the piezoelectric elements and the lower pressure plate by the insulating tube. The extension reaches the lower pressure plate and is clamped for fixing the center electrode, the piezoelectric elements, and the pressure plates in an assembled form. The outer periphery of the assembly is surrounded by insulating molded material, such as insulating synthetic resin, rubber and so forth. This insulating molded material also serves for retaining the components of the pressure sensor in the assembled form.
A washer-type pressure sensor is mounted on an engine cylinder block together with a spark plug which is screwed into in the engine cylinder block and inserted into the engine combustion chamber. The spark plug is subject to combustion pressure in the engine combustion chamber which varies the tightening torque. This variation of the tightening torque of the spark plug reflects variation of the combustion pressure in the combustion chamber. Therefore, the washer-type pressure sensor monitors the tightening torque exerted thereon and thus monitors variation of the internal pressure in the combustion chamber. Such a sensor is useful in carrying out various types of engine control, such as spark ignition control by detecting engine knocking, maximum pressure point and so forth.
However, in some conventional washer-type pressure sensors, the insulating material surrounding the sensor assembly are exposed to corrosive fluid, such as gasoline. Corrosion of the insulative material may cause shortening of the life of the washer-type pressure sensor.